This invention relates in general to a method for growing silicon crystals, and more particularly to a method for growing silicon crystals having a uniform and predetermined concentration of oxygen.
Most of the silicon used in the semiconductor industry is produced in single crystal form by the Czochralski technique. In this technique a quantity of pure silicon is melted in a silica crucible, a seed crystal is dipped into the silicon melt and then controllably withdrawn to pull a single crystal ingot from the melt. The silicon charge is usually very pure silicon to which controlled amounts of dopant have been added to yield a crystal having the desired conductivity type and resistivity.
Oxygen is an unintentional dopant impurity in Czochralski grown silicon crystals which results from the dissolution of the silica crucible used to contain the silicon melt. During the crystal pulling process oxygen enters the silicon lattice where it occupies an interstitial position. Interstitial oxygen in silicon can be either detrimental or beneficial depending upon the intended use of the silicon. One important beneficial result, in certain applications, is intrinsic gettering which results from a high interstitial oxygen concentration. To accomplish intrinsic gettering the oxygen concentration in the grown silicon is carefully controlled within a critical range, greater than about 1.6.times.10.sup.18 cm.sup.-3. During the subsequent processing of a silicon substrate, that oxygen is caused to precipitate in the bulk of the substrate while at the same time it is depleted by outdiffusion at the substrate surface. This results in a region of oxygen defects in the bulk of the substrate and a zone denuded of defects at the surface. During successive steps at elevated temperatures the oxygen precipitates attract highly mobile and usually electrically active metal contaminants away from the active device area at the substrate surface. The contaminants are immobilized at the oxygen precipitates leaving a low contaminant, high lifetime surface zone.
Unfortunately, the practice of intrinsic gettering is made difficult by the fact that oxygen is not uniformly distributed throughout a silicon ingot. Ingots grown by the Czochralski technique are typically characterized by an axial oxygen concentration distribution which is high at the seed end, increases still further within the first few centimeters of crystal, decreases gradually through most of the crystal and then rises again at the tang end. If the typical Czochralski ingot is sawed into a plurality of wafers, some of the wafers from that ingot will have the desired oxygen concentration for intrinsic gettering and others will not. To use these wafers in an intrinsic gettering application would therefore require that each wafer be characterized and sorted according to oxygen concentration. The result is an expensive and time consuming sorting operation and a low overall yield.
There is, therefore, a need for silicon wafers having a uniform oxygen concentration in a particular concentration range which cannot reproducibly be supplied by prior art techniques.
It is therefore an object of this invention to provide an improved process for the growth of silicon crystals.
It is another object of this invention to provide an improved process for the growth of silicon crystals having a uniform oxygen concentration.
It is yet another object of this invention to provide an improved process for the growth of high oxygen concentration silicon crystals.
It is still another object of this invention to provide an improved silica crucible for use in the pulling of a silicon crystal.
It is still another object of this invention to provide an improved process for growing silicon crystals having a uniform concentration of oxygen in a predetermined range.